Conventional automotive ignition timing control normally operates in an "open-loop" control mode in which ignition timing is determined through application of engine parameter information to an ignition timing schedule that is determined under calibration conditions. Such open loop control necessarily but erroneously assumes that engine operating conditions will not deviate significantly from the calibration conditions. Any variation in conditions away from those calibration conditions can result in ignition timing error and reduced engine performance. Knock sensors are known to provide some feedback information indicating engine performance that can be used, in extreme conditions, to limit ignition timing. However, unless a knock condition is detected by the knock sensor, open-loop control and its associated shortcomings will dictate engine cylinder ignition timing.
The ionization tendency of cylinder combustion gases has been exploited to diagnose cylinder knock conditions and cylinder misfire conditions. The ion content in the cylinder combustion plasma is known to indicate the character of a cylinder combustion event. Generally, the ion content is measured by applying a supplemental voltage across the electrodes of a spark plug during a cylinder combustion event to measure the current carrying capacity of the plasma to which the electrodes are exposed. The magnitude of the voltage waveform across the electrodes may then be analyzed to determine the quality of the combustion event, so that a misfire condition or a knock condition, for example, may be indicated.
The ionization current across the spark plug electrodes during a cylinder combustion event is proportional to cylinder mean effective pressure (MEP). Variation in MEP, and therefore variation in the ionization current has been determined to be at a minimum at an optimum ignition timing advance angle of a cylinder, as illustrated in FIG. 1. It would be desirable to apply ionization current information for ignition timing control to drive an open-loop ignition timing command toward an optimum ignition timing advance angle. However, ionization current is sensitive to a variety of conditions in addition to MEP, including fuel composition and temperature. The magnitude of ionization current, as is currently applied in knock or misfire diagnostics, is therefore poorly suited to direct application in engine ignition timing control.
It would be desirable to provide for a more comprehensive closed-loop ignition timing control than conventional knock-based ignition timing limiting. It would further be desirable to exploit cylinder ionization current information to determine optimum ignition timing advance angle of a cylinder. It would still further be desirable to control ignition timing on-line in response to such determined optimum ignition timing advance angle.